Generally, combustion turbines have three main assemblies, including a compressor assembly, a combustor assembly, and a turbine assembly. In operation, the compressor assembly compresses ambient air. The compressed air is channeled into the combustor assembly where it is mixed with a fuel. The fuel and compressed air mixture is ignited creating a heated working gas. The heated working gas is typically at a temperature of between 2300 to 2900° F. (1200 to 1593° C.), and is expanded through the turbine assembly. The turbine assembly generally includes a rotating assembly comprising a centrally located rotating shaft and a plurality of rows of rotating blades attached thereto. A plurality of stationary vane assemblies, each including a plurality of stationary vanes, are connected to a casing of the turbine and are located interposed between the rows of rotating blades. The expansion of the working gas through the rows of rotating blades and stationary vanes or airfoils in the turbine assembly results in a transfer of energy from the working gas to the rotating assembly, causing rotation of the shaft.
The combustor assembly typically includes a plurality of combustors arranged in an annular array about the engine. The hot working gas from each combustor is transferred to the turbine by a respective transition duct. The outlet of the combustor is generally cylindrical, and the inlet to the turbine at the exit to each transition duct generally corresponds to an arcuate sector. Accordingly, the cross-sectional shape of the transition duct must change from a generally cylindrical shape at the combustor exit to a generally rectangular arc-like shape at the turbine inlet. In addition, since the combustors are typically mounted at a radial outward location relative to the turbine inlet, the transition ducts must define a gas path extending radially inwardly in the direction of the gas flow to the turbine.
The combination of complex geometry changes as well as extreme mechanical and thermal loading seen by the transition duct create a harsh operating environment that can lead to premature deterioration, requiring repair and replacement of the transition duct. In particular, as higher firing temperatures are utilized, increased transition duct failures may occur due to low cycle fatigue (LCF) cracks that may be observed at an upper panel of the transition. Accordingly, extreme care must be taken with respect to the design of these geometric transitions to avoid sharp geometric changes that may create regions of high stress, i.e., stress concentration points.